Method for the production of precipitated calcium carbonate with a high degree of brightness

ABSTRACT

Calcium carbonate with high brightness is prepared by treating calcium carbonate and/or the milk of lime used for its preparation by reacting milk of lime with carbon dioxide and/or at least one of the calcium-containing preliminary products used for the preparation of the milk of lime before, during and/or after this reaction with a bleaching agent which comprises at least one compound of the formula (I): 
 
A[(CR 1 R 2 )SO p M (1/q) ] r   (I) 
where the variables have the following meanings: 
     A is NR 3 R 4 , NR 3 , N or OH;    R 1 , R 2 , R 3 , R 4  independently of one another, are hydrogen or an organic radical; M is ammonium or metal p is 2 or 3; q is the valency of M; and    r is 1 when A=OH or NR 3 R 4 , is 2 when A=NR 3  and is 3 when A=N; 
 
and where variables, if a mixture of compounds is used and/or r=2 or 3, are chosen independently of one another for each individual compound and/or for each [(CR 1 R 2 )SO p M (1/q) ] group.

The present invention relates to a process for the preparation ofprecipitated calcium carbonate with high brightness. Precipitatedcalcium carbonate (often referred to by the acronym “PCC”) is animportant inorganic mass-produced substance whose main field of use isuse as filling and coating pigment in papermaking. PCC is also used innumerous other fields of use, for example as pigment or filler incosmetics, plastics, paints and coatings.

In: “Lime and Limestone: Chemistry and Technology, Production and Uses”,Wiley-VCH, Weinheim 1998 (ISBN 3-527-29527-5), J. A. H. Oates gives areview of the chemistry and technology of limestone, lime and itssecondary products such as PCC. For the latter, the sections 12.9 (p.109, Use) and 31.2 (p. 352-354, Production) are particularly relevant.Limestone is the collective term for natural deposits of calciumcarbonate, which arises predominantly in the form of the mineralcalcite. Apart from limestone, marble (hydrothermally recrystallizedcalcite) and chalk (calcium carbonate sediment), which can be processedlike limestone for the purposes of the present invention, are also typesof these deposits. Lime is the collective term for certain limestonesecondary products, in particular the term includes the calcinationproduct of limestone, burnt lime (synonyms are calx, caustic lime, quicklime; chemically the material is calcium oxide) and the reaction productof burnt lime with water, slaked lime (in solid form this is alsoreferred to as hydrated lime, in the form of paste moistened with wateras lime putty and in aqueous suspension as milk of lime, chemically thematerial is calcium hydroxide). Calcium is often also substituted atleast partially by magnesium in limestone and its secondary products.

PCC is usually used when ground limestone (whiting) does not correspondto the profile of requirements for the given use. During the preparationof PCC, it is possible to influence, inter alia, morphology and particlesize distribution of the particles and their degree of contamination ina desired manner. On an industrial scale, PCC is usually prepared byreacting milk of lime with carbon dioxide. The processes usedindustrially typically start from burnt lime, which is reacted withwater to give milk of lime. This milk of lime is freed from particleswhich are too coarse and interfere with further processing, and is thenreacted with a carbon dioxide-containing gas (for example pure carbondioxide or a carbon dioxide/air mixture), in most cases in a series ofreactors with precise control of the pH, of the temperature and of thedegree of (super)saturation in order to establish the desired morphologyand particle size distribution of the PCC. The PCC suspension preparedin this way is either used as it is or the PCC is, in most cases afterexcessively coarse particles have been filtered off, isolated as a solidby customary methods for separating off solids from liquids (for examplefiltration, centrifugation, evaporation) and drying. The particle sizemay, if required for a given application, be changed in a customarymanner, for example by grinding or agglomeration. The process can beintegrated in various ways into limestone processing (for example usingthe CO₂-containing exit gases which are formed during lime burningand/or the combustion gases from the lime burning furnace, the solidproduct of which is used for the preparation of milk of lime) ormodified (for example by preparing milk of lime by dispersing hydratedlime in water). It is likewise possible to use calcium carbonate,calcium oxide or calcium hydroxide from sources other than limestone,for example carbide lime hydrate, a secondary product from theelectrothermal calcium carbide preparation from coke and limestone.

WO 01/17905 and the specification cited therein disclose processes forthe preparation of PCC.

An important quality feature of lime products is their brightness (morerarely whiteness). The brightness is measured in standardized processes,for example in accordance with DIN 5033 “Colorimetry”, Part 9 (1982) andDIN 53163 “Testing of pigments and fillers” (1988) or the correspondinginternational standards. For this, measurement is essentially made, in aphotometer, of the reflectance of light of a certain wavelength (in mostcases 457 nm) compared with a white standard determined as 100%reflectance (in most cases pure barium sulfate). The brightness isreduced by the impurities present in the limestone. Typical impuritiesof limestone are quartz, clay minerals, feldspars, pyrite, hematite,silica rocks, phosphates and sulfates (gypsum, anhydrite), thebrightness being impaired in particular by impurities which contain ironand manganese. During the preparation of PCC, the brightness increasescompared with the starting material limestone, inter alia as a result offiltering off impurities which are suspended in water, not like calciumhydroxide in milk of lime. Ground limestone from comparatively puredeposits therefore typically has brightnesses in the range from 75 to95, but PCC has brightnesses above 95.

The requirements on the brightness of PCC are high, particularly in thecase of use as filler or coating composition for papermaking. For thisreason, PCC is often also treated with bleaching agents.

According to the teaching of WO 96/20308, paper containing filler, inparticular PCC, is treated, during the pressing operation after theactual dewatering of the pulp, with a bleaching agent chosen fromhydrogen peroxide, sodium tetrahydroboranate and sodium hydrosulfite. WO85/05386 discloses a process of bleaching pulp using hydrogen peroxide,where magnesium carbonates, and also whiting are added, although not asfiller, but as basic auxiliary of the bleaching operation. WO 99/32710teaches a similar process using a peracid as bleaching agent.

EP-A-197 327 discloses a process for the preparation of PCC in whichammonium or amine salts are added to the milk of lime, after filtrationof the milk of lime, activated carbon is added, which is then likewisefiltered off again, and then the PCC is precipitated by introducingcarbon dioxide. WO 99/61374 teaches a process for the preparation of PCCwhere a suspension auxiliary is added to the precipitated PCC whichestablishes a positive zeta potential of the PCC at a pH of less than 9,for example a carboxylic acid. During the precipitation of the PCC, itis possible to add an auxiliary chosen from the group formed byhydrazine, hydroxyamine, soluble salts of hydroxyamine, solubledithionite salts and mixtures thereof.

However, it is also known to treat PCC with reducing bleaching agents.Such bleaching agents primarily convert the undesired iron and manganesecompounds into soluble iron and manganese salts. Thus, WO 99/61374teaches a process for the preparation of PCC in which sodium dithioniteis added to the milk of lime prior to precipitation. (Dithionites areoften also referred to as hydrosulfites.) U.S. Pat. No. 4,900,533discloses the precipitation of PCC in the alkaline pH range in thepresence of sodium dithionite or zinc dithionite in an amount of from0.45 to 9 kg per ton of PCC. This process has the disadvantage thatthese dithionites are unstable compounds which are only stable uponstorage to a limited degree and which are entirely capable of selfigniting, particularly when they come into contact with moisture, andare therefore difficult to handle. Furthermore, their thermalsensitivity limits the applicable temperature range. U.S. Pat. No.4,900,533 explicitly limits the temperature to be used in the processdisclosed therein to a maximum of 45° C. due to decomposition of thedithionites at higher temperatures. Although WO 99/61374 discloses ingeneral maximum temperatures up to 100° C., the actual operatingtemperature used therein is 40° C. This temperature limitationnecessitated by the instability of the dithionites prolongs the requiredreaction time and also makes bleaching during precipitation of the PCCimpossible since temperatures significantly above 50° C. typically ariseas a result of the exothermic nature of the precipitation, unless,something which is disadvantageous from an economic point of view, useis made of cooling devices.

JP-A-94/41 892 discloses a process for the bleaching of precipitated PCCor, in particular, calcium carbonate from the caustification of soda inwhich the PCC or calcium carbonate is treated in an alkaline medium withsodium or zinc dithionite, sodium tetrahydroboranate, sodium sulfite,sodium hydrogensulfite thiourea dioxide(synonym: formamidinesulfinicacid) or Rongalit (not specified in any more detail). Rongalit® is atrade mark of BASF Aktiengesellschaft and serves as a collective termfor designating various sulfinic acid derivatives which aredifferentiated by the further designation which follows the name. Theproducts sold under the trade name Rongalit are used in particular intextile dyeing and textile printing. For example, Rongalit C is sodiumhydroxymethanesulfinate dihydrate, Rongalit H is calciumhydroxymethanesulfinate, further products sold under this trade name areor were, for example, Rongalit ST, Rongalit DP, Rongalit DS, Rongalit 2PH-A, Rongalit 2 PH-B, Rongalit CL, Rongalit CW, Rongalit BA andRongalit FD (this list is not necessarily complete; thealkanesulfonates/-sulfinates are often also referred to asalkylsulfonates/-sulfinates, for example sodium hydroxymethanesulfonateis thus also referred to as sodium hydroxymethylsulfonate). Like in U.S.Pat. No. 4,900,533 and WO 99/61374, according to the teaching ofJP-A-94/41 892, sodium dithionite is used at 40° C.; alternatively,thiourea dioxide, which as is known can be used at a significantlyhigher temperature, is used at 80° C. However, due to the admittedly inmost cases low, but in practice often unavoidable, residual content ofthiourea, which is a substance with well-founded suspicion ofcarcinogenic potential, the use of thiourea dioxide for the bleachingencounters considerable opposition.

WO 98/03725 discloses, inter alia, compounds which can be described bythe following formula:A[(CR¹R²)SO_(p)M_((1/q))]_(r),where the variables have the following meanings

-   A is NR³R⁴, NR³, N or OH;-   R¹, R², R³, R⁴, independently of one another, are hydrogen or an    organic radical;-   M is ammonium or metal-   p is 2 or 3;-   q is the valency of M; and-   r is 1 when A=OH or NR³R⁴, is 2 when A=NR³ and is 3 when A=N;    and where these variables, if a mixture of compounds is used and/or    r=2 or 3, are chosen independently of one another for each    individual compound and/or each [(CR¹R²)SO_(p)M_((1/q))] group, and    also the preparation of such compounds and their use for reductive    aftercleaning during the dyeing of textiles containing polyester.

It is an object of the invention to find further simple andcost-effective processes for the preparation of calcium carbonate withthe highest brightness. We have found that this object is achieved by aprocess for the preparation of calcium carbonate with high brightness byreductive bleaching by means of at least one sulfinic and/or sulfonicacid derivative, which comprises treating calcium carbonate and/or themilk of lime used for its preparation by reacting milk of lime withcarbon dioxide, and/or at least one of the calcium-containingpreliminary products used for the preparation of the milk of limebefore, during and/or after this reaction with a bleaching agent whichcomprises at least one compound of the formula (I):A[(CR¹R²)SO_(p)M_((1/q))]_(r)  (I)where the variables have the following meanings:

-   A is NR³R⁴, NR³, N or OH;-   R¹, R², R³, R⁴ independently of one another, are hydrogen or an    organic radical;-   M is ammonium or metal-   p is 2 or 3;-   q is the valency of M; and-   r is 1 when A=OH or NR³R⁴, is 2 when A=NR³ and is 3 when A=N;    and where variables, if a mixture of compounds is used and/or r=2 or    3, are chosen independently of one another for each individual    compound and/or for each [(CR¹R²)SO_(p)M_((1/q))] group.

Surprisingly, bleaching with these sulfinic acid derivatives leads tosignificantly better brightnesses than can be achieved with hydrosulfitebleaching. Also, using the process according to the invention, the useof greater amounts of sulfinic acid derivative results in a greaterbrightness, in contrast to hydrosulfite bleaching in which higheramounts of bleaching agent no longer lead to a further increase inbrightness. In addition, with the process according to the invention, itis possible to work at relatively high temperatures withoutdecomposition of the bleaching agent, which increases the reaction rateof the bleaching and thus the space-time yield.

Compounds and compound mixtures of the formula (I) and their preparationare known. They are described, for example, in Ullmann's Encyclopedia ofIndustrial Chemistry, 5th Edition, Volume A 25, VCH VerlagsgesellschaftmbH (in the interim: Wiley-VCH Verlag GmbH), Weinheim 1994 (ISBN3-527-20100-9), keywords “sulfinic acid and derivatives” (pages 461-476)and “sulfonic acids, aliphatic” (pages 503-506), in WO 98/03725, and theliterature cited in these references, to the entire contents of whichexpress reference is hereby made.

The compounds of the formula (I) are, when p=2, derivatives of sulfinicacid and, when p=3, derivatives of sulfonic acid. If two or three[(CR¹R²)SO_(p)M_((1/q))] groups are bonded to a central nitrogen atom(A=NR³, N), and the degree of oxidation of the sulfur in these groups isdifferent (i.e. p is not identical for the individual(CR¹R²)SO_(p)M_((1/q))] groups in the molecule), mixedsulfinates/sulfonates are present.

In formula (I), A is NR³R⁴, NR³, N or OH. If A=OH and accordingly r=1then the compounds of the formula (I) are hydroxymethanesulfinates or-sulfonates. If A=NR³R⁴ and accordingly r=1, the compounds of theformula (I) are usually referred to asaminomethanesulfinates/-sulfonates, if A=NR³ and correspondingly r=2, asiminomethanesulfinates/-sulfonates, and if A=N and accordingly r=3, asnitrilomethanesulfinates/-sulfonates.

R¹, R², R³ and R⁴, independently of one another, are hydrogen or anorganic radical. A suitable organic radical is any radical inert underthe reaction conditions of the process according to the invention.Examples of suitable organic radicals are aliphatic, cycloaliphatic,aromatic, arylaliphatic or heteroaromatic radicals, such as alkyl,alkenyl, alkynyl, cycloalkyl, cycloalkenyl, aryl, such as phenyl,alkylaryl or arylalkyl. The radicals may be substituted by furtherorganic radicals and also include heteroatoms, for example in the formof alkoxy substituents, ester groups, amino or alkylamino functions. Theheteroatoms, such as oxygen, sulfur or nitrogen, may also be part of anaromatic or cyclic radical. The radicals can also be linked with oneanother and [lacuna] parts of single- or multi-numbered, mono- orpolycyclic ring systems, for example having 4 to 16 carbon atoms, whichinclude the incorporated radicals R¹ and R² and the carbon atom of the[(CR¹R²)SO_(p)M_((1/q))] group in question and/or the incorporatedradicals R³ and R⁴ and the N atom of the NR³R⁴ group in question.Examples of suitable organic radicals R are C₁-C₂₀-alkyl radicals, inparticular C₁-C₆-alkyl radicals, and C₃-C₈-cycloalkyl radicals, wherethese (cyclo)alkyl radicals may be substituted by one to threeC₁-C₄-alkyl radicals, such as methyl, ethyl, 1-propyl, 2-propyl,1-butyl, 2-butyl, 1-(2-methyl)propyl, 1-(1,1-dimethyl)ethyl, orcorrespondingly linear or branched pentyl, hexyl or longer-chainradicals, such as, for example, eicosyl, or cyclopropyl, cyclobutyl,cyclopentyl, cyclohexyl, cycloheptyl, cyclooctyl radicals, which may, ifdesired, also be substituted by alkyl radicals. In a preferred manner,the radicals R¹, R², R³ and R⁴ are hydrogen or C₁-C₄-alkyl, inparticular methyl or ethyl.

M is ammonium or metal of valence q. Ammonium is R¹R²R³R⁴N, where theabove definitions apply for the radicals R. Ammonium is preferably NH₄⁺. Suitable metals are in principle any metal which forms stablesulfinate or sulfonate salts. In particular, the metals used are thealkali metals Li, Na, K, Rb and/or Cs, the alkaline earth metals Mg, Ca,Sr and/or Ba and/or Zn. Preferred metals are Na, Ca and Zn.

q is the valence, i.e. the ionic charge of the metal present naturallyin the form of a cation in the compounds of the formula (I), or ofammonium. For ammonium and the alkali metals, q=1, for the alkalineearth metals and zinc, q=2. If other metals are used, q may also behigher. If metals of different valence are used in a mixture, q is to beinserted specifically into the formula (I) for the metal in question.The charge balance—each (CR¹R²)SO_(p) group naturally carries a singlenegative charge—can, if q is not 1, take place intramolecularly within amolecule according to the formula (I), such as, for example, inN[(CH₂)SO₂)₃NaCa, but also intermolecularly between two or moremolecules according to the formula (I), in the latter case two or moremolecules according to the formula A[(CR¹R²)SO_(p)M_((1/q))]_(r) sharean actual metal ion, as, for example, in [N(CH₂SO₂)₃]₂Ca₃. Thefirst-mentioned example compound here corresponds to the general formula(I) as N[(CH₂)SO₂(Na_(1/1)Ca_(1/2))]₃, and the second-mentioned compoundcorresponds to the formula (I) as N[(CH₂)SO₂Ca_((1/2)]) ₃.

The variable p is 2 or 3. If p=2, it is a sulfinate or a sulfinategroup, and if p=3, it is a sulfonate or a sulfonate group.

In industry, such compounds are prepared easily in a known manner byreacting dithionite S₂O₄M_(2/q) or hydrogensulfites HSO₃M_(1/q) withketones R¹—CO—R² or aldehydes (R¹—CO—H or formaldehyde, R¹=R²═H).

In this process, hydroxymethanesulfonates and -sulfinates according tothe formula (I) and specifically according to the formulaHO—CR¹R²—SO_(p)M_(1/q) are formed. These sulfonates and sulfinates are,if desired, then reacted with ammonia, primary amines R³NH₂ or secondaryamines R³R⁴NH. This reaction produces, with the elimination of water,condensation products of the formula (I) in which the hydrogen atom(s)on the ammonia or amine nitrogen is/are completely or partially replacedby —CR¹R²—SO_(p)M_(1/q) groups. The stoichiometry of the feed substancespredetermined during the preparation and the simultaneous addition orobservance of an order of addition determine, in a quite obvious manner,the ultimate compounds and the ultimate compound mixture. The content ofsulfinate and sulfonate groups can be readily determined analytically,for example by redox titration. Typical reaction conditions are atemperature of generally at least 20° C., preferably at least 40° C. andgenerally at most 100° C., preferably at most 90° C., and a reactiontime of generally at least 15 minutes, preferably at least 30 minutesand generally at most 10 hours, preferably at most 6 hours, and anaqueous reaction medium. The reaction pressure applied is not important,and atmospheric pressure is generally used.

The sulfinates can also be obtained by reduction of the correspondingsulfonates, for example by catalytic reduction of the sulfonates withhydrogen or by reaction thereof by means of zinc dust. The partialreduction of sulfonates is a further method for the preparation of mixedsulfonates/sulfinates.

For some compounds of the formula (I), further special preparationprocesses are known. Thus, for example, sodium hydroxymethanesulfinateHO—CH₂—SO₂Na is usually prepared by reacting sulfonyl chloride withsodium amalgam or sodium formate. Sodium hydroxymethanesulfinate, whichis normally in the form of the dihydrate, is also a common commercialproduct and is available, for example, under the name Rongalit C fromBASF Aktiengesellschaft, Ludwigshafen, Germany, or in various commercialforms under the name Brüggolit® from L. Brüggemann KG, Heilbronn,Germany.

It is entirely possible to prepare the compound(s) of the formula (I)only in situ, i.e. in the presence of the PCC, the milk of lime or thecalcium-containing feed substances used for their preparation.

Compounds or compound mixtures of the formula (I) preferred for theprocess according to the invention are i) sodium hydroxymethanesulfinateand ii) the mixture of nitrilomethanesulfinate/-sulfonates producedduring the reaction of three equivalents of a sodiumhydroxymethanesulfinate/-sulfonate mixture with ammonia.

The bleach to be used in the process according to the inventioncomprises at least one compound of the formula (I). Further componentscan be added to the bleach, for example fillers and/or auxiliaries, suchas, for example, drying agents, buffer substances, antidust agents,fragrances for masking unpleasant odors, stabilizers and the like. Suchadditives are known for reducing agents based on sulfinate and sulfonatefrom the technology of textile dyeing and can also be used here. Inparticular, in the case of the treatment according to the invention ofcalcium carbonate with a bleaching agent, it may be advantageous to adda complexing agent to the bleaching agent. In this connection,preference is given primarily to citric acid and/or oxalic acid and/ortheir salts, in particular their alkali metal and/or alkaline earthmetal salts, for example their sodium, potassium, magnesium and/orcalcium salts. Although an addition of dithionite is possible in theory,it brings the abovementioned disadvantages of the PCC bleaching withdithionite and is therefore avoided in the normal case. It is, however,also possible to add a pure compound of the formula (I) or a mixture ofcompounds of the formula (I) as bleaching agent, and in this case thebleaching agent thus consists of one or more compounds of the formula(I). It is likewise possible to use as bleaching agent a mixture which,in addition to one or more compounds of the formula (I), comprises onlyunavoidable impurities and/or components which have been deliberatelyadded to an immaterial extent, such as, for example, auxiliaries. Inthis case, the bleaching agent thus consists essentially of one or morecompounds of the formula (I).

In the process according to the invention, calcium carbonate and/or themilk of lime used for its preparation by reacting milk of lime withcarbon dioxide, and/or at least one of the calcium-containingpreliminary products used for the preparation of the milk of lime istreated before, during and/or after its reaction with a bleaching agentwhich comprises at least one compound of the formula (I), thereby beingreductively bleached. (In the text below “bleaching agent” is to beunderstood as meaning a bleaching agent which comprises at least onecompound of the formula (I).) For example, calcium carbonate (natural orsynthetically prepared calcium carbonate such as PCC), burnt lime ormilk of lime is treated with bleaching agent. In the solid state (in thecase of calcium carbonate or burnt lime), the bleach is used in dryform, for example by mixing with the solid to be bleached or by commongrinding. In the liquid state (calcium carbonate suspension or milk oflime), the bleaching agent is usually added to the suspension and/or tothe suspending agent. The bleaching agent can, however, also be added tothe solid, which is then suspended, or in the case of burnt lime,slaked. In the case of burnt lime, although bleaching in a suspension inan inert (therefore necessarily nonaqueous) suspending agent (forexample hydrocarbons) is possible, it is generally economicallyuninteresting.

In a preferred embodiment of the process according to the invention,calcium carbonate is treated with bleaching agent. The origin of thecalcium carbonate and its purity are unimportant. For example, groundlimestone, marble, chalk or PCC is used. In the process according to theinvention, as in the processing of calcium carbonate, it is expedientand generally customary to initially establish the particle size desiredfor the given application of the calcium carbonate by grinding, unlessthe crude product used is already of this size. If a grinding step isnecessary, the calcium carbonate is treated with bleaching agent before,during or after this grinding step. In most cases, the treatment of thecalcium carbonate with bleaching agent before a necessary grinding stepis inappropriate since in most cases a lesser increase in the brightnessis obtained for the finished product than if the calcium carbonate istreated before or after the grinding step. It is therefore preferable,if a grinding step is necessary, to add the bleaching agent during thegrinding step, otherwise the calcium carbonate is treated directly withbleaching agent. For this, the calcium carbonate can be ground or mixedin the dry state with the bleaching agent, although preference is givento treating a suspension of the calcium carbonate with bleaching agent.The suspending agent used is a liquid, preferably water. If a grindingstep is necessary, it is easier to carry it out as a wet grinding stepand to add bleaching agent during the wet grinding step.

In another preferred embodiment of the process according to theinvention, the reductive bleaching is combined with the preparation ofcalcium carbonate, in particular the precipitation of PCC by reaction ofmilk of lime with carbon dioxide. For this purpose, bleaching agent isadded to the reaction mixture before, during and/or after theintroduction of a carbon dioxide-containing gas into milk of lime. Forexample, bleaching agent is added to the milk of lime prior to theintroduction of a carbon dioxide-containing gas. Preferably, some of thebleaching agent is added to the milk of lime prior to the introductionof carbon dioxide, and a further part of the bleaching agent is added tothe reaction product following precipitation of the PCC. In thisembodiment, it is necessary to work in suspension since both the milk oflime and also the reaction mixture are suspensions during and after theintroduction of carbon dioxide. The suspending agent is a liquid,preferably water.

The solids content of the suspension is chosen in both embodiments outof cost considerations such that it is neither necessary to circulateunnecessarily high amounts of liquid, nor to treat a paste which canonly be stirred and conveyed with very great difficulty. Typical solidscontents of the suspensions are in the range from at least 3% by weightof calcium carbonate, based on the total mass of the suspension,preferably at least 5% by weight and, in a particularly preferredmanner, at least 10% by weight, and generally at most 90% by weight,preferably at most 50% by weight and in a particularly preferred mannerat most 20% by weight. The solids content of the milk of lime used inthe process according to the invention combined with the PCC preparationis correspondingly chosen so that said calcium carbonate solids contentsare achieved after the reaction.

In the case of the treatment of the calcium carbonate or of the milk oflime with bleaching agents, excessively low pH values are normallydisadvantageous since calcium carbonate decomposes in the acidic rangewith the evolution of carbon dioxide. Generally, in the case of thetreatment of already precipitated PCC, a pH of at least 5, preferably atleast 6 and not more than 9, preferably not more than 8, is established.In the case of PCC precipitation, the initial pH of the milk of lime isstrongly basic and drops over the course of the precipitation. Thebleaching before or during the PCC precipitation therefore takes placeat the pH present in the precipitation mixture.

It is possible to work at a low temperature; the temperature isgenerally more than 10° C. or preferably more than 20° C. Typically,however, the reaction rate here is also low, which reduces thespace-time yield. In a particularly preferred manner, the temperature istherefore at least 50° C. It is also generally at most 95° C. andpreferably at most 90° C. It may be advantageous to add a complexingagent to the suspension if one was not present in the bleaching agentused. If this procedure is used, preferred complexing agents are citricacid and/or oxalic acid and/or their salts, in particular their alkalimetal and/or alkaline earth metal salts, for example their sodium,potassium, magnesium and/or calcium salts. With the exception of thebleaching temperature, which tends to be higher, and the use of thebleaching agent according to the invention, all of these are alsocustomary parameters and measures for the industrially common PCCpreparation or for the known hydrosulfite bleaching of calciumcarbonate.

The concentration of bleaching agent used according to the invention andthe treatment time are chosen so that the desired increase in thebrightness is achieved, without unnecessarily high amounts of bleachingagent being consumed or an unnecessarily large amount of time beinglost. In general, bleaching agent is added in an amount such that atleast 0.1 gram of compound of the formula (I) or mixture of compounds ofthe formula (I) are used per kilogram of calcium carbonate, preferablyat least 0.5 g/kg and in a particularly preferred manner at least 0.7g/kg, and generally at most 200 g/kg, preferably at most 150 g/kg and ina particularly preferred manner at most 100 g/kg. A decisive aspect inthis respect is the content of sulfinate groups in the bleaching agent.For a bleaching agent which has a lower relative proportion of sulfinategroups, the amount to be used is accordingly greater than for ableaching agent with a higher relative proportion of sulfinate groups.The treatment time is generally at least 15 minutes, preferably at least30 minutes and in a particularly preferred manner at least one hour andgenerally at most 10 hours, preferably at most 8 hours and in aparticular preferred manner at most 6 hours. In the embodiment in whichthe process according to the invention is combined with theprecipitation of PCC, it is usually not necessary to add these reactiontimes to the typical reaction times for the PCC preparation. It sufficesto choose a total reaction time of PCC precipitation and bleaching whichis sufficiently long.

If the bleaching agent is added in the process according to theinvention combined with the PCC preparation at two or more timeintervals, the amount of bleaching agent used overall is divided into anumber of portions which are added at these different time intervals.For example, part of the total amount of sulfinic acid derivative isadded to milk of lime at a temperature of at least 10° C., preferably atleast 20° C. and at most 90° C., preferably at most 50° C., then acarbon dioxide-containing gas is introduced, thereby precipitating PCC.The partial amount of bleaching agent used here is an arbitrary partialamount of the overall amount of bleaching agent. It is usually generallyat least 10% by weight of the total amount of bleaching agent,preferably at least 20% by weight and in a particularly preferred mannerat least 30% by weight, and generally at most 90% by weight, preferablyat most 80% by weight and in a particularly preferred manner at most 70%by weight. For example, 50% by weight of the total amount of bleachingagent are added to the milk of lime before introducing a carbondioxide-containing gas. The remaining bleaching agent is added after theintroduction of gas is complete. It is, however, likewise possible toadd the total amount of bleaching agent in three parts before, duringand after the introduction of gas, and a division into more than threeportions and also a continuous addition of bleaching agent are likewisepossible.

EXAMPLES

In accordance with the procedures (a), (b) and (c) described below, (a)unbleached PCC from Schaefer Kalk KG, Diez, Germany (the product whichis commercially available in bleached form as PRECARB® 100) with aninitial brightness of 94.20 was treated with the bleaching agent to beused in accordance with the invention, (b) PCC was prepared byintroducing a carbon dioxide-containing gas into a milk of lime treatedbeforehand with bleaching agent, and (c) PCC was freshly prepared frommilk of lime by reaction with carbon dioxide and then treated withbleaching agent.

All of the brightness measurements were carried out on samplespulverized in a mortar in accordance with a modification of DIN 53145 inwhich measurements were made not on compacts, as specified therein, buton unpressed powder. The brightnesses measured are therefore too lowcompared with brightnesses measured on compacts.

The experiments were carried out on the one hand with the mixture ofnitrilomethanesulfinate/-sulfonates (“mixture”, Experiments 1-12) formedin the reaction of three equivalents of a sodiumhydroxymethanesulfinate/-sulfonate mixture with ammonia, and on theother hand with sodium hydroxymethanesulfinate (“Rongalit C”,Experiments 13-27). The Comparative Experiments C1-C12 were carried outwith sodium hydrosulfite (“hydrosulfite”) as bleaching agent.

The procedures were:

Procedure (a): 100 g of a PCC slurry in water (15% by weight of PCC) aretreated at 60° C. with the bleaching agent and stirred. After 4 hours,the solid is filtered off and dried.

Procedure (b): 100 g of milk of lime (11% by weight of calcium hydroxidein water) are treated, with stirring, with the bleaching agent, then aCO₂/air mixture [50% by volume of CO₂, 40 l/h] is introduced, thetemperature increasing slowly over 1 h from the starting value 30° C. to65° C. After a further 1 hour at 65° C., the solid is filtered off anddried.

Procedure (c): A CO₂/air mixture (50% by volume of CO₂, 40 l/h] is thenintroduced into 100 g of milk of lime (11% by weight of calciumhydroxide in water), during which the temperature is slowly increasedover 1 h from the starting value 30° C. to 65° C. The mixture is thentreated, with stirring, with the bleaching agent. After a further 1 hourat 65° C., the solid is filtered off and dried.

The bleaching agent concentrations used are given in Table 1, in eachcase in grams of bleaching agent used per 100 g of calcium carbonate.The amounts used were chosen so that, at a given concentration c1, c2,c3, c4 or c5, identical molar equivalents of sulfinate (i.e.reducing-active molecules or groups) are present in the bleaching agentsused. TABLE 1 Concentration Hydrosulfite Mixture Rongalit C c1 0.37 0.971.24 c2 0.74 1.95 2.48 c3 1.11 2.93 3.6 c4 1.48 3.66 4.96 c5 2.22 5.847.44

Parameters and results of Examples 1-27 and Comparative Examples C1-C12are summarized in Table 2 below. “n.d.” means: “not determined”: TABLE 2(Comparative) Bleaching Example No. Procedure agent Conc. Brightness C1 a Hydrosulfite c1 95.4 C2  b Hydrosulfite c1 95.3 C3  c Hydrosulfite c195.7 C4  a Hydrosulfite c2 95.3 C5  b Hydrosulfite c2 94.8 C6  cHydrosulfite c2 96.0 C7  a Hydrosulfite c4 95.0 C8  b Hydrosulfite c495.7 C9  c Hydrosulfite c4 95.8 C10 a Hydrosulfite c5 95.0 C11 bHydrosulfite c5 95.7 C12 c Hydrosulfite c5 95.9  1 a Mixture c1 90.7  2b Mixture c1 95.6  3 c Mixture c1 96.1  4 a Mixture c2 91.0  5 b Mixturec2 95.7  6 c Mixture c2 96.7  7 a Mixture c4 92.2  8 b Mixture c4 95.2 9 c Mixture c4 95.8 10 a Mixture c5 93.7 11 b Mixture c5 95.4 12 cMixture c5 96.8 13 a Rongalit C c1 91.4 14 b Rongalit C c1 96.0 15 cRongalit C c1 96.6 16 a Rongalit C c2 91.8 17 b Rongalit C c2 96.7 18 cRongalit C c2 96.5 19 a Rongalit C c3 n.d. 20 b Rongalit C c3 95.8 21 cRongalit C c3 97.0 22 a Rongalit C c4 95.8 23 b Rongalit C c4 96.5 24 cRongalit C c4 97.0 25 a Rongalit C c5 95.4 26 b Rongalit C c5 n.d. 27 cRongalit C c5 n.d.

The examples show that in the case of the bleaching of PCC (examplesusing procedure (a)) with the process according to the invention, thebrightness of the PCC can, in contrast to the prior art (hydrosulfite)be increased by higher concentrations of the bleaching agent, and in thecase of higher concentrations, the process according to the invention(in particular in the case of the use of Rongalit C) leads tosignificantly better brightnesses of the PCC than the prior art. Theexamples also show that in the case of the addition of bleaching agentto the milk of lime prior to precipitation of the PCC (examples usingprocedure (b)) using the process according to the invention, significantincreases in the brightness are achieved compared with the prior art(bleaching of the PCC with hydrosulfite). They also show that also inthe case of freshly precipitated PCC (examples using procedure (c)), theprocess according to the invention leads to greater brightnesses thanthe prior art.

1. A process for the preparation of calcium carbonate with highbrightness by reductive bleaching by means of one sulfinic and/orsulfonic acid derivative, which comprises treating calcium carbonateand/or the milk of lime used for its preparation by reacting milk oflime with carbon dioxide, and/or at least one of the calcium-containingpreliminary products used for the preparation of the milk of limebefore, during and/or after this reaction with a bleaching agent whichcomprises at least one compound of the formula (I):A[(CR¹R²)SO_(p)M_((1/q))]_(r)  (I) where the variables have thefollowing meanings: A is NR³R⁴, NR³, N or OH; R¹, R², R³, R⁴,independently of one another, are hydrogen or an organic radical; M isammonium or metal p is 2 or 3; q is the valency of M; and r is 1 whenA=OH or NR³R⁴, is 2 when A=NR³ and is 3 when A=N; and where variables,if a mixture of compounds is used and/or r=2 or 3, are chosenindependently of one another for each individual compound and/or foreach [(CR¹R²)SO_(p)M_((1/q))] group.
 2. A process as claimed in claim 1,wherein the bleaching agent is added to the milk of lime prior to itsreaction with carbon dioxide.
 3. A process as claimed in claim 2,wherein bleaching agents are added both to the milk of lime prior to itsreaction with carbon dioxide, and also to the calcium carbonatesuspension prepared by the reaction of milk of lime with carbon dioxide.4. A process as claimed in claim 1, wherein the amount of bleachingagent added is such that at least 0.1 gram of compound or mixture ofcompounds of the formula (I) are used per kilogram of calcium carbonate.5. A process as claimed in claim 1, wherein the treatment with bleachingagent is carried out at a temperature in the range from at least 50 toat most 90° C.